INTRODUCTION TO AMINES AND THEIR PHYSICAL PROPERTIES

What are amines?

The easiest way to think of amines is as near relatives of ammonia, NH3.

In amines, the hydrogen atoms in the ammonia have been replaced one at a time by hydrocarbon groups. On this page, we are only looking at cases where the hydrocarbon groups are simple alkyl groups.

The different kinds of amines

Amines fall into different classes depending on how many of the hydrogen atoms are replaced.

Primary amines

In primary amines, only one of the hydrogen atoms in the ammonia molecule has been replaced. That means that the formula of the primary amine will be RNH2 where “R” is an alkyl group.

Examples include:

Naming amines can be quite confusing because there are so many variations on the names. For example, the simplest amine, CH3NH2, can be called methylamine, methanamine or aminomethane.

The commonest name at this level is methylamine and, similarly, the second compound drawn above is usually called ethylamine.

Where there might be confusion about where the -NH2 group is attached to a chain, the simplest way of naming the compound is to use the “amino” form.

For example:

Secondary amines

In a secondary amine, two of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. At this level, you are only likely to come across simple ones where both of the hydrocarbon groups are alkyl groups and both are the same.

For example:

There are other variants on the names, but this is the commonest and simplest way of naming these small secondary amines.

Tertiary amines

In a tertiary amine, all of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. Again, you are only likely to come across simple ones where all three of the hydrocarbon groups are alkyl groups and all three are the same.

The naming is similar to secondary amines. For example:

 

Boiling points

The table shows the boiling points of some simple amines.

typeformulaboiling point (°C)
primaryCH3NH2-6.3
primaryCH3CH2NH216.6
primaryCH3CH2CH2NH248.6
secondary(CH3)2NH7.4
tertiary(CH3)3N3.5

The Amine Structure

Amines contain a nitrogen atom with a lone pair of electrons. They are basically the derivatives of ammonia. If we replace one or more hydrogen atoms by substituent groups such as alkyl or aryl in Ammonia, we get a new class of organic compounds called Amines. Examples of significant amine structure include Amino acids, biogenic amine, trimethylamine, and aniline.

Physical properties of amines

Classification of Amines

Amines are classified into four categories namely, primary, secondary, tertiary and cyclic. A primary amine is one when you can replace one of the three hydrogen atoms by an alkyl or aryl group. When you can replace two out of three hydrogen atoms, we get secondary amine.

Similarly, when all the three hydrogen atoms are replaced tertiary amines are formed. Only secondary or tertiary amines can be cyclic. The 3 – member ring aziridine is an example of cyclic amine. Let us now look at the physical properties of amines in brief.

Physical Properties of Amines

  • The lower aliphatic amines are gaseous in nature. They have a fishy smell.
  • Primary amines with three or four carbon atoms are liquids at room temperature whereas higher ones are solids.
  • Aniline and other arylamines are generally colourless. However, they get coloured when we store them in open due to atmospheric oxidation.
  • Lower aliphatic amines can form hydrogen bonds with water molecules. Therefore, such amines are soluble in water.
  • Increase in the size of hydrophobic alkyl part increases the molar mass of amines. This usually results in a decrease in its solubility in water.
  • Higher amines are insoluble in water. Organic solvents like alcohol, benzene and ether readily dissolve amines.
  • Alcohols have higher polarity as compared to amines and hence, they form stronger intermolecular hydrogen bonds.
  • Primary and secondary amines are often engaged in the intermolecular association as a result of hydrogen bonding between the nitrogen of one and hydrogen of the other molecule.
  • The intermolecular association is more prominent in case of primary amines as compared to secondary due to the availability of two hydrogen atoms.
  • In tertiary amines, there is no intermolecular association due to the absence of free hydrogen atom for bonding.
  • The order of boiling point of amines is as follows: Primary > Secondary > Tertiary.

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